Demystifying few terms
Windows 8: This is the flagship product of Microsoft and has undergone many a version changes and alongside feature, functionality changes in the last 3 decades or so. Latest in the series of client operating systems, Windows 8 brings significant changes in user experience, keeping pace with the current generation innovations including tablet devices and touch interfaces. There are multiple versions of Windows 8 that you need to check out to decide which one to buy in case you are upgrading to it. It has a new modern UI way or working and a desktop that is same as Windows 7. It can run all existing applications as is. It can connect to the Windows 8 Store and download modern UI based applications and run. Note that the store applications don't run in desktop mode and vice versa.
Windows RT: While Windows all these years has mostly been on Intel chips, with the smartphone and tablets, ARM chips have got a significant boost. These chips are lot more power friendly, as is a key requirement for a mobile device. Windows RT is Windows 8 targeted towards PCs, laptops and tablets that run on ARM chips. Unlike Windows 8 however, Windows RT isn't something that you can buy across the counter. It is only available as pre-installed OS on a new PC or tablet. You may want to read this FAQ for more details. It has a version of Office software written specifically to run on ARM. Other than that it can connect to Windows 8 Store to download apps and run. Regular Windows 7 applications will not run on this.
Metro UI/Modern UI: Microsoft introduced live tiles based user experience with Windows Phone 7. The same was extended and provided in Windows 8. The new design language that included full screen content, no chrome, fast and fluid responses etc. became to be known as Metro. However the name was changed midway and is now called as Modern UI. This has led to lot of confusion as Metro UI as a term had gained lot of momentum.
Win RT: A new design language, a new user experience, also meant a new programming platform and that is what WinRT is. It stands for Windows runtime. It is the new platform using which applications for modern UI of Windows 8 can be written. It supports creation of applications using HTML5/jQuery or XAML/C#/VB.NET. Application written targeting WinRT are uploaded to Windows 8 Store and any Windows 8 or Windows RT device can connect and download it and run it.
Surface: Earlier the name of the multi-touch table like hardware from Microsoft, the name was recently re-used to refer to Windows 8 tablets. The erstwhile Surface is now called as Pixel Sense. Surface RT is a tablet running on Windows RT and is ARM based and Surface tablet is one running Windows 8 and runs on Intel chip. The second one can be treated like a full blown laptop itself. Surface running Windows 8 Pro (one of the editions of Windows 8), is also called as Surface Pro. The models available today are mostly based on ARM. See some of them listed here.
Side loading: This is neither hardware related nor to the Windows 8 user experience, but more on how to deploy applications on a Windows 8 machine. The new WinRT based applications for Windows 8 are to be downloaded from the Windows 8 Store, but in case of organizations we don't expect that all of their applications will go live on Windows 8 Store. There are many applications that will be internal to any organization and will not go on Windows 8 Store. The ability to install these applications on a Windows 8 device, without having to access the store, is called side loading. This feature will be enabled on Windows 8 Enterprise Edition only and administrators will typically push such applications or create an Enterprise hub (managed via Windows Intune) for people to connect and download apps from. If you can unlock a device using development ID, then you can install any WinRT based app on it, without having to go to the store, if you have the APPX (application package) file with you.
Please share if you have any more terms that need explanation.